, 1887-02-16

UC6u3 IcCOIuCu DIS averages " OuC UIhCO
for summer temperature, winter tempera-
tnre,, and yearly temperature. BY joining
on 1.-The maps the places which had the same
average yearly temperature, lines were
produced passing round the world mainly
in an eastern and western direction. These
were calletf isothermal lines. A map of
the world was exhibited showing the iso-
therms for annual averages, as we'll as seve.
raj summer and winter isOtherms. In re-
gard 10 the first named it was shown that
they were much more irregular i'll the
northern hemisphere than in the southern,
a fact which was due to the great masses
Of land which diversified the northern
hemisphere. 1t was shown that the
isotherms assembled towards the north as
they passed over the oceans from west to
east, while they descended towards the
south when traced across the continents in
the same direction, facts which were TO be
explained by the prevailing winds and
ocean currents.
A very interesting diagram was shown
OF the average maximum and minimum
temperatures of each month traced
throughout the year for Santa Cruz and
for Copenhagen. The difference Of climate
of the two places was thus brought before
the eye in q striking manner.
The lecturer then proceeded to speak Of
the moisture in the air. He said thai if
water were brought TO a certain temperature,
namelyHtheI boiling point, it would pass off
into the air. in, steam. The air, however,
3 did not get its moisture i'll this way, but by
evaporation. he amount OF evaporation
varied much in different places according
to the conditions of the air. In Cumana, S.
America, it amounted to several yards in q
year, in London to only about % inches.
When the air at amy place had absorbed
as much moisture as it could hold, no more
evaporation could take place, and if the
air were then cooled, "d0wnpOur" would
take place, that is the air would give up
its moisture in the shape Of rain, snow, hail
or dew. The formation Of clouds was also
explained. The warm air from the carth'S
surface assembled to the higher cooler air
and some of its moisture was consequently
given up i'll the form Of minute hollow balls
Of water. These formed the clouds. The
amount OfmOiSture in the air was meas lred
by taking two thermometers, and wrapping
the bulb Of one in cloth connected with q
small reservoir of water so that it was kept
constantly wet. When the air was dry the
evaporation around the bulb was great and
in the process heat was absorbed from the
bulb, consequently the mercury fell. Ii on
the other hand the air were saturated with
moisture no evaporation wOnld take
place and the wet bulb thermometer would
consequently remain at the same height as
the dry bulb. Thus the less the difference
in the two Thermometers the greater was
the dampuesS of the air.
The weight of the atmosphere was mra.
sured by a barometer, which consisted of
glass tube closed at the top, turned up
atthe bottom and filled with mercury. The
mercury was, sustained in The tube by the
pressure OF the air. WheH the pressure
increased the barometer went up, when it
decreased the barometer fell. Dry heavy
air caused it to rise; moist and consequent.
IL light air caused it to fall. There WAS
another kind of barometer called the
GNerOjd. Essentially this consisted of two
plates of metal fitted into Il rim and enclOS.
ing an air tight space. When the atmOS.
pheric pressure increased the plates were
pressed towards elch other, when the pres.
sure decreased they sprang ont again. This
motion of the plates was then transferred
by a simple arrangement to an index on a
plate resembling u dial plate.
The lecturer proceeded to describe the
laws of storms and hurricanes, q part of
The lecture which was very interesting,
but could scarcely be reproduced without
the aid of diagrams. We hope to see this
part of The lecture expanded some day into
a separate lecture on hurricanes, which
would, we are sure, he both iuterrSt ting and
valuable in an island which lies, as this
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